Recently, interest in the energy storage technology has been increased. The effort to research and develop an electrochemical device has been gradually materialized as the application field of the energy storage technology has been expanded to a mobile phone, a camcorder, a notebook PC, and an electric vehicle. The electrochemical device is a field which attracts the most attention in this respect, and in particular, the development of a secondary battery capable of being charged and discharged is the focus of attention.
Among the secondary batteries which are currently applied, the lithium ion battery developed in the early 1990s has been widely used as a power source of portable apparatuses since it was developed in 1991 as a small battery, a light-weight battery, and a large capacity battery. The lithium secondary battery is in the spotlight due to its advantages that the operating voltage is higher and the energy density is far greater as compared to batteries of prior art, such as a Ni-MH battery, a Ni—Cd battery, and a sulfuric acid-Pb battery which use an aqueous electrolytic solution. In particular, the lithium secondary battery is mainly adopted as a medium- or large-sized battery with an energy unit of kWh or more used for electric vehicles and energy storage, and a cathode active material which has a high capacity and is usable for a long period of time is desired for this purpose.
The full-scale commercialization of large capacity secondary batteries containing manganese spinel (LMO) and olivine-based cathode materials (LFP) which exhibit excellent thermal stability is inhibited due to a low energy density thereof, and thus the application of a layered type cathode material having a high capacity is increasingly required for an improvement in battery properties. A layered type cathode material among the cathode materials for lithium secondary battery can realize the highest capacity among the materials which are currently commercialized. The use of LiCoO2 that is frequently used in a small IT apparatus such as a smart phone in a medium- or large-sized battery is inhibited by the problems of safety, a low capacity, the economic efficiency due to a high cost and limited resource due to the reserves of cobalt metal of a main raw material as compared to other transition metals, the environmental regulations due to the environmental pollution, and the like. A number of researches on LiNiO2 which has the same structure as LiCoO2 have been carried out for the advantages that its price is relatively inexpensive and can have a high theoretical capacity of 200 mAh/g. However, LiNiO2 has not been commercialized due to the problems such as poor stability and drastic deterioration in lifespan by structural instability generated when being produced.
In order to improve the disadvantages of LiNiO2, a part of nickel is substituted with a transition metal element so as to slightly shift the temperature at which the heat generation starts to a higher temperature or to prevent drastic heat generation, and other measures are attempted. The material, LiNi1-xCoxO2 (x=0.1 to 0.3), obtained by substituting a part of nickel with cobalt exhibits relatively excellent charge and discharge characteristics and lifespan characteristics as compared to LiNiO2 but still does not exhibit sufficient lifespan performance. In addition, a number of technologies related to the composition and production of a Li—Ni—Mn-based composite oxide obtained by substituting a part of Ni with Mn which exhibits excellent thermal stability or a Li—Ni—Mn—Co-based composite oxide obtained by substituting a part of Ni with Mn and Co are known, and a new-concept cathode active material has been recently disclosed in Japanese Patent Application Laid-Open No. 2000-227858 in which not LiNiO2 or LiMnO2 is partially substituted with a transition metal but Mn and Ni compounds are uniformly dispersed in the atomic level to form a solid solution.
According to European Patent 0,918,041 or U.S. Pat. No. 6,040,090 on the composition of a Li—Ni—Mn—Co-based composite oxide obtained by substituting Ni with Mn and Co, LiNi1-xCoxMnyO2 (0<y≤0.3) exhibits improved lifespan performance and thermal stability as compared to an existing material composed of only Ni and Co but still has problems to be solved, such as poor thermal stability and deterioration in lifespan performance as a Ni-based material.
In order to solve this disadvantage, a patent on a lithium transition metal oxide having a concentration gradient in the metal composition is proposed in Korea Patent Application No. 10-2005-7007548.
However, by this method, a high capacity can be realized as the cathode active material is synthesized so as to have different metal compositions in the inner layer and the outer layer, but the metal composition is not continuously and gradually changed in the cathode active material thus produced. In addition, the interface between the inner layer and the outer layer may act as a resistant component to lower the output and to deteriorate the lifespan performance when the cathode active material is used for a long period of time, and a difference in concentration gradient is not substantially generated at a high temperature for heat treatment of 850° C. or higher due to the thermal diffusion of metal ions and thus the effect of performance improvement is insignificant.